Corrosion inhibiting



Patented Feb. 3, 195a 2,872,281 CORROSION INHIBITING Harry L. Kahler, Feasterville, and Charles B. George, Philadelphia, Pa, assignors, by mesne assignments, to Betz Laboratories, Inc., a corporation of Pennsylvania No Drawing. Application March 24, 1954 Serial No. 418,458

11 Claims. (Cl. 21-2.7)

The present invention relates to new and improved corrosion inhibitory compositions and to new and improved processes for inhibiting corrosion.

The present application is related to Kahler and George U. S. patent application Serial No. 390,916, filed November 9,-1953, for Corrosion Inhibiting, now U. S. Patent No. 2,793,932, granted May 28, 1957.

The compositions of the present invention are particularly effective in preventing or reducing corrosion produced on metal surfaces in contact with water. Such compositions are particularly applicable to cooling towers,

air-conditioning systems, condensers, heat exchangers,

and distributing systems where water constitutes the corroding me dium and ferrous or nonferrous metals or both constitute the material susceptible to corrosion attack.

It has been known for many years that various water soluble phosphates will inhibit water corrosion under certain conditions and extensive use has been made of such phosphate compounds. However, in order to use phosphates effectively, it is necesary to adjust their dosage to such factors as velocity of the water stream, temperature, chemical composition of the water, and other added ingredients. Thus, higher water velocities require lower phosphate concentrations, higher water temperatures require higher phosphate concentrations, and higher concentrations of chlorides, sulphates and other similar ions necessitate the use of higher concentrations of phosphate. Because of this rather direct influence of environmental factors on the effectiveness of phosphates in inhibiting corrosion, some investigators, aware of erratic results, have concluded that their behavior could not be pre dicted in any given 'case and that accordingly the phosphates must be considered somewhat unreliable inhibitors for general use. The same investigators class phosphates as dangerous inhibitors (P. Hamer and L.

Powell, Metallic Corrosion, Passivity and Protection; U. R. Evans, Corrosion (2d edition, Longmans, Green and Company) The present inventors have investigated phosphates alone and have found them to have inferior inhibitory post, ease of use, and the by-product advantage of sequestering power for the reduction of scale. Likewise considerable effort has been directed to improving the corrosion inhibitory behavior of phosphates.

One of the present inventors has taken steps in that direction by combining phosphates with chromates, as set forth in co-pending application Serial No. 364,871, filed June 29, 1953, for Corrosion Protection in Water Systems, now U. S. Patent No. 2,711,391, granted June 21, 1955.

l The present inventors in a continued study to obtain further improvement in corrosion inhibitory treatments for use in water media, have discovered that various additive materials can be used with phosphates or compositions containing phosphates so that greatly improved in hibitory power is obtained. We have also discovered a y in contact with the water.

new process for utilizing these additive materials to obtain pronounced improvements in effectiveness of results and economy.

A purpose of the present invention is to obtain new and improved corrosion inhibitory compositions. I

A further purpose is to provide a water treating composition of improved inhibitory power which derives a part of its effectiveness from the presence of a water soluble phosphate compound, such as a water soluble complex or molecularly dehydrated phosphate, or a water soluble orthophosphate or a combination of both, used with a water soluble trivalent chromium compound. The materials of this soluble composition are introduced into the corrosive water in such a manner that the solubility is exceeded with resultant deposition on the metal surfaces The deposit is adherent and very protective against corrosive water. The highly insoluble compounds of the deposit are intended to include only those resulting from the reaction of the treatment materials while they are in the main body of the cooling water. Accompanying reactions of corrosion at the water-metal interface to produce coatings on the metal surfaces are purely coincidental and, we believe, not beneficial, as weight losses of metals are too small to be significant after the protective coating has been deposited. Since this is true for many difierent metals, we believe that the coating mechanism is in no way associated with the corrosion processes.

A further purpose of the invention is to provide a composition of improved inhibitory power containing a water soluble phosphate compound, preferably a complex or molecularly dehydrated phosphate, or an orthophosphate, or a combination of both together, with a water soluble chromate compound containing hexavalent chromium, and a water soluble trivalent chromium com- 7 pound. The materials of this composition are introduced into the corrosive water in such a manner that compounds are developed in excess of their solubility with the resultant precipitation of a coating on the metal surfaces in contact therewith. The highly insoluble compounds of the deposit are intended to include only those resulting from the reaction of the treatment materials. discussed above with the chromate and the phosphate. Concomitant reactions of corrosion to produce coatings on metal surfaces are purely coincidental and no benefits appear to accrue from the same.

A further purpose of the invention is to provide a water-treating composition of improved corrosion inhibitory power containing a water soluble phosphate, preferably a complex or molecularly dehydrated phosphate, or an orthophosphate, or a combination of both together, with a water-soluble chromate compound containing hexavalent chromium and a water-soluble molybdate compound containing hexavalent molybdenum.

A further purpose is to employ for corrosion inhibition in a water solution from 1 to 1000 p. p. m., and preferably from 1 to 200 p. p. m., of water-soluble phosphate compound, preferably complex phosphate, along with from 0.1 to p. p. m., preferably from 0.2 and 25 p. p. m.,- of trivalent chromium ion and to maintain .a pH in the water between 3 and 8.5 and preferably between 5 and 8.2.

A further purpose is to inhibit corrosion in an industrial water system and thus protect the metallic parts by introducing into the water from 1 to 1000 and preferably from 1 to 200 p. p. m. of water-soluble phosphate compound, preferably complex phosphate, along with 1 to 1000 p. p. m., preferably 1 to 200 p. p. m., of watersoluble chromate compound, and along with from 0.1 to 100 p. p. m. and preferably from 0.2 to 25 p. p. m. calculated as metal ion of a compound of the class which consists of water solublecompounds of trivalent chromiumand water-soluble com-pounds hexavalent-molybdenum, and to maintain a pH range between 3 and 8.5 and preferably between and 8.2 in the trivalent chromium ion and between 5 ar1d""9*and'- preferably between 5. 5 and 825' in the case of 'hexavalentmolybdenum-ion.

A further purpose is to accompany any one of theab'ove treatments with the addition'of an 'o'rga'nic reducing agent of the class which consists of sugars, alcohols, lig'nins andtannins in a concentration of l to 1000"p. p. in.- and preferably 5 w'z'oop. p. in.

"-Thus, it ''will' be" evident that in the invention we ernploy a phosphate, preferablya nrolecularly dehydrated phosphate or less des'irablyan 'o'rth'ophosphate,- and that the phosphate may be} used withanyone of thefollowing additional materials:

1 A water 1 soluble trivalent chromium compound. '(2) A chromate arid-awatersoluble trivalent chrominim-compound.

-(-3') A chromate-anda*hexavtlent molybdenumcompound.

(-4) A chromate, atrivalent'chromium compound-and a hexavalent molybdenum compound.

Thus, it has been found that exceptional results are realized when a phospha'te'is combinedwith'a watersoluble chromium compound, with a chroma'te compound and atrivalent chromium compound, or 'witha-ehromate compound and a hexavalent molybdenum compound, or with -a chromate compound,*a trivalent chromium compound and a hexavalent molybdenum compound. The advantageous features of using-'chromate alon'g'with trivalent chromium ion are includedin our-copending appl'ication- Serial No. 390,916 above referred to; now UJ S. Patent 27 93.932.

V'TO *il-lust'ra'te the benefits Of 5 IlieSWi'IIIPFOVGdCQIFOSlOH inhibitory treatments; the results of -aseries of 1 evaluatory tests-are submitted herewith. In this 'se'riie's, tests'were made ona'ground surface high carbon' steei specimen of normal corrodib'ilit'y' to permit a comparison and evaluation 'of'ir'ihibitory functions and -powers. The several corrosion-specimens were examined "to evaluate pitting particularly as set forth in cop'e'nding application 'serial No. 364,'8 7l,"n'ow U. S. Pate'ntNO. 1,711,391.

Table I illustrates the various types-of waters-used. Analysis of the Philadelphia tap water involvedis as follows:

Calcium 50p: p. m. as C5 C0 Magnesium '30 p. p. m.- as CaCO Bicarbonate alkalinity 30 p. p. m. as CaCQ Chloride 14 p; p. m.

Sulphate 30-p.p;m.

'The medium hardness, low chloride'water'is a Phi1adelphia tap water of the above analysis with added ioris to increase calcium to 110 to 130 p. p. m., magnesium to 40 to 50 p.. p. m., and bicarbonate alkalinity to'110 to 130 p. p. m., all calculated as calcium carbonate, and to increase sulphate ion to 70'to 80p. p. m.

TABLE I Type Water Composition II Philadelphia tap whterWitli-KOO p. p. In. chloride ion added. III Philadelphia tap water with 500 pp. rn. chloride ion added Iv and 300' p. p. m sulphate ion added.

Medium hardness, low chloride water as set forth above.

Table II sets forththe varioustreatment compositions which were tested' eiip'ressedin-terms of the materials indicated at the headings of the columns. In treatment composition I thefi? DJ: p. ma of hexavalent molybdenum ion is equivalent to'15 p. p. m. of sodium molybdate.

"TABLE II Sodium. Disodiuur 7 Sodium -Cl1ro- -Molyh- Treatine'nt Ohro- .Phos; 'Tri-Poly 'mi'um as denum Composimate, 'phate, fiphos- Gr as Mo, tions p.-p.m. p. p. m.-- phage, prpun. ppzrn.

The test results are given in-TableI-II, which gives the resultof one daytests on speci-mens having-a test area of Theapparatus-used for the laboratory-experiments was a---cor-1 tinuousflowing experimental water system "divided into two sec-tio ns, one--seetion of specimens reeeiving the water having the-inhibitorand-the othersection of specimens receiving the controlwater. This system -therefore permitted the testing of one controhand one treatment experimentsimultaneouslyor two controls or two-treatment experiments. Downstream from the points of addition ofthe' inhibitor corrosion test specimens were exposed to the water. The flow rate used during the tests was 0.-35 foot per second and the temperature was 120 F. The specimens Were-fiatsteel sheets havinga composition of 0.9% carbon,-l.4%--manganese, 0.04% sulphur,- 0.3% silicon,-no phosphorus, balance iron. Before exposure-thespecimens were resurfaced using a No. grit grinding wheeltoa machined surface of R. 8.20 micro-inches to allow for accurate evaluationoflpits.

Beforeimmersion, the specimens were cleaned with tripoli.(an abrasive) and trisodium phosphate followed by a water-rinse, an. alcohol rinse, and drying. The oxygen in the test water waskept at '5 p. p. m. The specimens were evaluated at -a -magnification of 20 diameters.

TABLE III 'Treat- Type Wt. Coat Products Distribu- TestNo. ment Water pH (III loss) (Ppted.) (Go'rrosion) 1 TubercIes Attack tion None IV 7.4 Overall. None :II 6.0 i Do.

A II 6.0 Wide.

B II 6.0 .8 D0.

0 IV 6. 0 Yesfblack Pitting D IV 6.0 Few Tiny.. Tiny Pits andShal- E III 6.0 1 do No Couple Fitting Trace.

F II 6.0 5 None Brown-Thin Numerous Shallow Pits Numerous.

G II 6.0 3 Blue-Green. Some Some.... do- 'Moderate.

11 II 6.0 5 one Brown Thin- Couple Tin Few.

I II 6.0 121 As Test No.2 No apparent above. difierence. I .0 II 6.0 30- None. Redandbleck.-Ineipient1..=.@-. GeneraLOverall..." P H 6.0 10 do do do GeneralRestricted..

As previously explained, phosphates have been of uncertain value as inhibitors over their entire permissible range of concentration. Chromates, on the other hand, although possessing a high susceptibility to localized attack, pitting and tuberculation, have possessed the power to completely stifle attack when used in adequate concentrations. p. p. m. of water-soluble chromate compound offer fair protection depending upon the corrosion load.

Tests in the series shown in Table HI and Table IV were made withtreatments formulated at concentration levels below adequate inhibition to provide a margin for judging improvements with the new compositions. The results shown are offered to illustrate the benefits of the invention, and not as suggestive of concentrations and formulations which would be recommended as fully adequate, and also not with the purpose of limiting the disclosure,

Example .1

In a corrosive water of type II, a treatment consisting of 26 p. p. m. of sodium tripolyphosphate and 26 p. p. m. of sodium orthophosphate and 2.0 p. p. m. of trivalent chromium ion in the form of chrornic chloride was found to exhibit remarkably effective corrosion inhibitory be havior. Whereas the use of water-soluble phosphate alone without the chromium ion decreased metal loss from 126 milligrams to 28 milligrams in one day, a treatment with phosphate and trivalent chromium ion as above set forth reduced the weight loss to one milligram. It is evident that this improved behavior was specifically due to the combination in view of the results of Table III, test IV, wherein trivalent chromium ion alone was used and the metal loss under the same conditions was reduced only. to 37 milligrams. In addition to the improvement in metal savings, the phosphate-chromate ion combination further prevented the formation of corrosion products and vastly reducedthe incidence of tuberculation and pitting. I

Example 2 Atreatment consisting of p. p. m. of sodium tripolyphosphateand 5 p. p. m. of sodium chromate and 2.0 p. p. m. of trivalent chromium ion in the form of trivalent chromium chloride was subjected to a test similar to that of Example 1. It produced remarkably effective corrosion inhibition, providing a reduction in weight loss from 126 milligrams in a control test to 3 milligrams under the same conditions in accordance with the invention. Although a treatment with p. p. m. sodium chromate alone gave a high metal saving, as did a treatment with 10 p. p. m. of sodium tripolyphosphate and 5 p. p. m. sodium chromate, the addition of the trivalent chromium ion reduced the incidence of pitting as well as preventing the continuation of localized attack which would lead to early perforation of the metal. The few pits which did occur when trivalent chromium ion was used with phosphate'and chromate as above, were more widely dispersed and more shallow than when chromate alone was used or phosphate and chromate were used as above set forth, and with 'no increase in metal loss.

Example 3 A treatment consisting of 10 p. p. m. of sodium tripolyphosphate'with 5 p. p. m. of sodium chromate and 2 p. p. m. of sodium molybdate calculated as hexavalent molybdenum ion, gave remarkably good results in inhibiting corrosion by reducing metal loss on one day test from 126 milligramsto 5 milligrams. In comparison to 10 p. p. m. of sodium tripolyphosphate and 5 p. p. m. of sodium chromate, the combined treatment with phosphate-chromate-molybdenum, although not showing added effectiveness in metal saving, greatly reduced the incidence of tuberculation and pitting, so as to improve gfatly the effectiveness of the treatment.

Minimum concentrations of 200 to 1000 Using lower concentrations of 5 p. p. m. sodium tripolyphosphate and 2.5 p. p. in. sodium chromate, the

addition of 2.0 p. p. in. sodium molybdate calculated as hexavalent molybdenum ion was very effective in reducing metal loss. This low concentration of phosphate plus chromate (without molybdenum) reduced metal loss from 126 milligrams to milligrams on one day test, but was not capable of fully preventing pitting attack. The addition of 2 p. p. m. of hexavalent molybdenum ion reduced the metal loss on one day test to 10 milligrams, and improved protection against pitting, while reducing the formation of corrosion products. It is evident that this combination itself was responsible for the good results, since Table III, test 13 using as much as 15 p. p. in. sodium molybdate alone was almost completely ineffective in reducing metal loss or otherwise improving the attack pattern.

In the examples given, we have shown the benefits containing soluble trivalent chromium ion, that benefits phosphates and chromates.

are derived through the application of a protective method heretofore never associated with treatments containing The use of phosphate and chromate treatments as previously practiced has required their complete solubility at all times, with particular emphasis on avoiding any form of. precipitation such as tricalcium phosphate. Any minor benefits. which might have resulted from precipitation of calcium phosphate coating are more than offset by difficulties of operation due to reduction in flow, decrease in heat transfer, and lack of uniformity of protection throughout the system.

On the other hand, the use of a coating type of inhibitory.

behavior such as is realized through the presence of trivalent chromium ion is remarkably beneficial. As will be noted in Table III, the trivalent chromium ion was responsible for producing a light blue or blue-green precipitated coating which was directly instrumental in giving improved corrosion inhibitory power over formulations of similar materials which lacked this coating agent. The use of this character of coating for corrosion protection is believed to be novel. The presence of phosphate with this coating is an improvement since it converts the coating from one which is primarily chromic hydroxide to one which includes chromic phosphate and is superior in adherence, uniformity of appearance, and effectiveness in preventing corrosion.

It has been found that formulations containing trivalent chromium best function in the pH range of 3 to 8.5 which constitute the insolubility limits of the chromium precipitates which are formed. Below a pH of 3 the chromic ion remains soluble, while above a pH of about 8.5 it exhibits its amphoteric character and is again soluble. Levels of concentration to provide the coating which can be included in effective formulations vary extensively. Better results in the form of better coatings are obtained in the preferred pH range of 5 to 8.2.

Phosphates employed should be water-soluble pho-sphate compounds, preferably molecularly dehydrated or complex phosphates such as sodium tripolyphosphate, sodium decaphosphate, sodium hexam-etaphosphate, em dium tetraphosphate, and corresponding potassium, lithium, ammonium and other Water soluble salts, which are hardness; temperature, and amountcf. phosphate ifiany precipitated with. thechromium.

The chromate usedmaybe any water-soluble chromate, such "assodiunrchromate, sodium 'dichromate; potassium tion of. anequivalentamount of chromate with subsequent reduction. to chromium-1 ion. in the mainwatersystem by reducing substances. such .-as hydrogen sulphide, sulphur dioxide, 1 and. organic ingredients.

chromate,. potassium dichromate, and ;corresponding 5 Inna series'of. tests with water of type III, in which water-soluble chromateanddichromate salts -of lithium hydrogenvsulphide concentrations of 1.-.9"to 3.4 p..p. m. and. ammonium. andother. Water-soluble chromates and were=present,: the useof the. coating. type of protective diohromatesand. chromic anhydride. The quantity in treatment-as-developed.bytrivalent'chromium was shown the water system willtvarybetween 1 and 10.00;p. p. m. to be particularlveffective. Table IV gives a series of and. preferably between land 200 pup-m. calculated as-v 10. these results, which established. the power. of. trivalent the water-soluble. chromate compound. There is no chromium ion whenpresent in-anadequate concentrapi'oblem. of. solubility-involved in .ordinary waters,- and tion. aT-he test period was-one day, using ground surtheconcentration of chromate. will be largely influenced facerhi'ghcarbonsteel specimens withsurface under test by economic considerations. of 3:x /z' x.- /e".

TABLE IV Treat- Typo H Wt. Coating Products I Distri- Test No. ment Water pH (hoss (Ppted) (Corrosion) TuberclesL- Attack button III 6.0 247 None 'Black',Heavy No' General 'Overall. III 6 .0 26 White N0 .Yes III 6.0 4 III 6.0 108 N0 Overall. III 6.0 -Few.Tin y 3.2 p. p. m. Or+++ produced by rductionoflo p. p. m..-NaiCr04..

The trivalent chromium compound may be. any water. soluble chromic compound and any water-soluble chromous compound which through ease of oxidation under the conditions of use-will be converted to chromic ions- Chromic chloride, nitrate and sulphate are examples. Suitablechromous salts are the acetate and chloride. The concentration of trivalent chromium ion will vary between'O.1' p. p. m. and'100. p. p. m. andinthe preferred embodiment between 0.2 p. p. m. and p. p. m.

Waters containing substantial contents of hydrogen sulphide presentaspecial corrosion problem. Asa result. of this-investigation, it has been found that. the-coatingv type treatment. using phosphate combined with trivalent chromium ions, is particularly effective in waters. contain.- ing reducing agents such as hydrogen sulphide. The use ofphosphates alone has been singularly unsuccessful in such waters, showing; less inhibiting power than in normal nonreducing water systems. Chromates alone in such systems when used without; phosphates are subject to rapid and almost complete reduction to trivalent chromium. As a result, the chromate eitherentirely disappears or is reduced in concentration to such a low value that its effectiveness as a chromate is materiallydecreased. The maintenance of adequate levels of chromate concentration in such circumstances is prohibitively expensive and often leads only to maintaining an. abundance of chromic ion which is not controlled, with attendant excessive, precipitation, sludging and scale formation', which interferes with normal operation. of the system.

[The chromic ion in itself has been shown in earlier discussions to be of limited corrosion inhibitory power.

The use of phosphates with chromates, a combination which has. considerable corrosion inhibitory power in reducing waters, is subject to the limitation that'the chro mate. is reduced and attempts to maintain the required chromate residual result in the same difficulties which have. been experienced with chromate alone.

The. present. inventors have found that where. reducing waters are. involved, the use of phosphates alone with trivalent chromium ion is a very effective corrosi ominhibitor. Iusuch treatrnenhthe chromic ion is introduced directly or is rapidly developed by reducing chromate.

The chromium concentration canthen be controlled and maintained; with provision for a controlled precipitation. of-chromium as a protective coating. 'Thetrivalent chromium. ion can'be introduced-as a soluble. trivalent chro- ITest=No. 3 in 'Fable I'V used=a treatment in which 312 p..'p; ':m. of trivalent chromium 'ion was produced in the :by the fact that higher weightrl'osses occurred when only 1 p. p. m.. trivalent chromium. ion wasv present.. .Table IV, test. 3,, showslth'at the .trivalent.chromium ion obtai'nd by reduction of chromateis just as effective as trivalent chromium ions .introducedas such in.test'-2.

Chromate. compounds can be used as sources of trivalent. chromium ion when-they act with reducingagents. The concentrations of chromates for such. purposes are determined onthe basis of'equivalents' of trivalent chromium ion over the trivalent. chromium. ionrange from:

.0.1' to 100 p. p. in.

beet sugar, molasses, methyl alcohol,,ethyLalcohol, propyl alcohol, isopropyl. alcohol, any of the alcohols mentioned in BernthsenfOrganic Chemistry (1923),. pages 68 to 86,, and. 446 to 448,. and any of the commercial forms of tannin and' lignin. When such organic reducingagents are present, it is possible to feed chromates with phosphates to produce a treatment which is a combination of phosphate, chromate and trivalent chromium ion. Such a treatment is very 'effective, as illustrated in Example:2..

=The presence .of'. such chromate-reactive organicreducing. agents. exerts: ben'efitsbeyond the-mere formation of trivalentchromiumnion- LSuchan. organic material used. with, phosphate and chromate, while: still present in'the. unreacted state, begins. to. cleanLthe. metal surfaces. and. solubilize sludgeand. corrosion products. In .addition, in cases. wherethe. calcium. concentration .is-high, and therefore the. phosphate. concentration. must. be.-maintained low. increased contents. of such. organic .materials augment. thapower'. or. the it'reatmentsivhich would other mium compound I or it can. be produced by t'he. introducwise be obtained from' chromates. with lowered phosphate concentrations. Such organic reducing agents are valuable adjuncts to the chromic coating type of corrosion inhibition. The concentration levels' of such organic reducing agents are not critical, and since they are not subject to the restrictions imposed on phosphates by calcium and magnesium, they can be used in the range from 1 to 1000 p. p. m. andpreferably from to 200 p.p.m. w

In contrast to the treatments containing trivalent chromium ion which use the coating mechanism for protection, molybdenum compounds are eifective in the soluble state to aid in inhibiting corrosion when used with phosphates and chromates. Hexavalent molybdenum in the form of molybdates of sodium orthe like is very soluble, and protective coatings of molybdenum are not obtained under the test conditions. The addition of molybdate compound to water-soluble phosphate alone was not found to be appreciably efiective. When added to a combination of phosphate and chromate within the ranges specified, however, vastly improved corrosion inhibitory behavior resulted. As shown by the tests, solubility of the molybdate compound is not a problem, and its efiectiveness improves with increased concentration, so that levels of 0.1 to 100 p. p. m. of hexavalent molybdenum ion are used with success, and preferably in the range from 0.2 to 25 p. p. m. The molybdates are the most satisfactory materials to feed for reasons of suitability, availability and cost, suitable compounds being sodium molybdate, potassium molybdate, corresponding salts of lithium and ammonium, other water soluble molybdates, molybdic acid, sodium, ammonium and other alkali metal polymolybdates, ammonium and other alkali metal phospho-molybdates, and other suitable water-soluble hexavalent molybdenum compounds used under conditions in which the compound remains soluble. Molybdenum compounds of lower valences, although relatively insoluble in water, can be eifectively used if they oxidize readily 'to the hexavalent form or are chemically altered to such form.

The water-soluble phosphate compounds, the watersoluble chromate compounds, and if desired, the watersoluble trivalent chromium compounds, can be eifectively used with the molybdates in the concentrations above set forth.

In view of our invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of my invention without copying the process and structure shown, and we therefore, claim all such insofar as they fall within the reasonable spirit and scope of our claims.

Having thus described our invention,-What we claim as new and desire to secure by Letters Patent is:

1. The process of protecting the metal parts in a cooling water system against corrosion, which comprises flowing water in a stream through the cooling water system and maintaining the water continuously in contact with the metal parts, adding to the water between 1 and 1000 p. p. m. of water soluble phosphate compound, between 1 and 1000 p. p. m. of water soluble chromate compound, and between 0.1 and 100 p. p. m. calculated as metal ion of a water soluble compound of the class which consists of compounds of trivalent chromium and compounds of hexavalent molybdenum, while maintaining a pH in the range between 3 and 8.5 in the case of trivalent chromium and in the range between 5 and 9 in the case of hexavalent molybdenum, and depositing on the metal parts a continuously replenishing protective coating.

2. The process of claim 1, in which the phosphate is a complex phosphate.

3. The process of claim 1, which comprises adding to the water between 1 and 200 p. p. m. of the water soluble -:-.phosphate compound, between 1 and .299 P, p. m. of

the water soluble chromate compound and between 0.2 and 25 p. p. m. calculated as metal ion of the water soluble compound of the cfass which consists of compounds of trivalent chromium and compounds of hexavalent molybdenum, while maintaining a pH in the range between 5 and 8.2 in the case of trivalent chromium and between 5.5 and 8.5 in the case of hexavalent molybdenum.

4. The process of claim 1 wherein: the water circulated contains a naturally occurring sulphur containing reducing agent whereby the hexavalent chromium is reduced to trivalent chromium.

5. The process of claim 1 wherein the water circulated contains a naturally. occurring reducing agent of the class consisting of sugars, alcohols, tannins and lignins.

6. The process of claim 1, which comprises also adding to the Water between 1 and 1000 p. p. m. of an organic reducing agent of the class consisting of sugars, alcohols, tannins and lignins.

7. The process of protecting the metal parts in a cooling water system against corrosion, which comprises flowing water in a stream through the cooling Water system and maintaining the water system continuously in contact with the metal parts, adding to the water between 1 and 1000 p. p. m. of water soluble phosphate compound, between 1 and 1000 p. p. m. of water soluble chromate compound, and chromium compound which on solution gives rise to between 0.1 and p. p. m. of trivalent chromium ion, while maintaining a pH in the range between 3 and 8.5, depositing on the metal parts a protective coating and continually replenishing the protective coating.

8. The process of claim 7 which comprises adding to the water between 1 and 200 p. p. m. of water soluble phosphate compound, between 1 and 200 p. p. m. of water soluble chromate compound, and chromium compound which on solution gives rise to between 0.2 and 25 p. p. m. of trivalent chromium ion, while maintaining a pH in the range between 5 and 8.2.

9. The 'process of claim 7, which comprises also add ing to the water between 1 and 1000 p. p. m. of organic reducing agent of the class consisting of sugars, alcohols, tannins and lignins.

10. The process of protecting metal parts in a cooling water system against corrosion, which comprises flowing water having a naturally occurring sulphur containing reducing agent therein in a stream through the cooling water system and maintaining the water continuously in contact with the metal parts, adding to the water between 1 and 1000 p. p. m. of a water soluble chromate compound, while maintaining a pH in the range between 3 and 8.5, reducing the water soluble chromate compound to trivalent chromium ion by said sulphur containing reducing agent in the water, and thereby building up in the water a concentration of trivalent chromium ion between 0.1 and 1000 p. p. m., depositing on the metal parts a protective coating and continuously replenishing the protective coating.

11. The process of protecting metal parts in a cooling water system against corrosion, which comprises flowing water in a stream through the cooling water system and maintaining the water continuously in contact with the metal parts, adding to the water between 1 and 1000 p. p. m. of a water soluble phosphate compound, adding to the water between 1 and 1000 p. p. m. of a water soluble chromate compound, while maintaining a pH in the range of 3 and 8.5, reducing the water soluble chromate compound to trivalent chromium ion by an organic reducing agent of the class consisting of sugars, alcohols, tannins and lignins, and thereby building up in the water a concentration of trivalent chromium ion between 0.1.and 100 p. p. m., depositing 0!} l}? metal a1 1 12 partseaprotective coating and continuously replenishing 368,619 -G'reat2 Britain Mar.;.1'0,.-.1932, theprotectivecoating, 647-;408 Great v-Bnitain', -Aug.26, 1942' 227 825 Switzerland .Oct. .1, 11943 References Cited inthefile of this patent 698,699 France -Feb. 3, 1931 UNIT Dv TA E .PATENT 5 7 E s T S OTHERREEERENCES 2,126,430 'Un'ger ---Aug; 9}1938 I v 9994 a e 2121942 :kobertsonc Electrochemwal 806., V51. '98} N0. 53, 2 73 17 Burns Man 30; 1954. Ch 1951, PP- 9 (P- 94 has act)- Jmudemiilk; Oihand: 1., December 7, .1946, pp. FOREIGN PATENTS 10v10541 enos a y.

Great Britain Feb. 13, 1940 

1. THE PROCESS OF PROTECTING THE METAL PARTS IN A COOLING WATER SYSTEM AGAINST CORROSION WHICH COMPRISES FLOWING WATER IN A STREAM THROUGH THE COOLING WATER SYSTEM AND MAINTAINING THE WATER CONTINOUSLY IN CONTACT WITH THE METAL PARTS, ADDING TO THE WATER BETWEEN 1 AND 1000 P.P.M. OF WATER SOLUBLE PHOSPHATE COMPOUND, BETWEEN 1 AND 1000 P.P.M. OF WATER SOLUBLE CHROMATE COMPOUND, AND BETWEEN 0.1 AND 100 P.P.M. CALCULATED AS METAL ION OF A WATER SOLUBLE COMPOUND OF THE CLASS WHICH CONSISTS OF COMPOUNDS OF TRIVALENT CHROMIUM AND COMPOUNDS OF HEXAVALENT MOLYBDENUM, WHILE MAINTAINING A PH IN THE RANGE BETWEEN 3 AND 8.5 IN THE CASE OF TRIVALENT CHROMIUM AND IN THE RANGE BETWEEN 5 AND 9 IN THE CASE OF HEXAVALENT MOLYBDENUM, AND DEPOSITING ON THE METAL PARTS A CONTINUOUSLY REPLENISHING PROTECTIVE COATING. 